This invention relates to a piezoelectric transducer with an adjustable resonance frequency and to an ultrasonic motor using such a piezoelectric transducer.
In particular, this invention relates to a vibration control unit for controlling mechanical vibration occurring with a mechanical component, and further to a vibration control unit for controlling a frequency distribution, an amplitude and other conditions of mechanical vibration.
Conventionally, an electromechanical transducing element uses an electrostrictive element, a magnetostrictive element or a piezoelectric element, and transduces an electric signal into mechanical vibration, thereby causing ultrasonic vibration. A piezoelectric transducer comprises the electromechanical transducing element provided with a resonator so that ultrasonic vibration with a large amplitude can be obtained.
In the related-art piezoelectric transducer, the configuration and structure of the resonator should be adjusted to obtain suitable resonance frequency for various purposes.
Japan Published Unexamined Application No. 63-125100 proposes a piezoelectric transducer in which the resonance frequency is adjustable. As shown in FIG. 5. in a piezoelectric transducer 50, a control piezoelectric element 54 having a control electrode 53 and a driven piezoelectric element 56 having a driven electrode 55 are stacked on top end of an elastic member 52 functioning also as a ground electrode. An elastic member 58 is arranged via an insulator 57 on top of the driven piezoelectric element 56. The piezoelectric transducer 50 is fastened with a bolt 59 and a nut 60. A direct-current power source 61 with variable voltage is connected to the control electrode 53. On the other hand, the driven electrode 55 is connected via a matching circuit 62 to a drive power source 63. Direct-current voltage from the direct-current power source 61 connected to the control electrode 53 is applied to the control piezoelectric element 54 of the piezoelectric transducer 50. Known piezoelectric elements contract and expand in proportion to applied voltage. Since the control piezoelectric element 54 expands and contracts in proportion to the applied direct-current voltage, the fastening force of the bolt 59 onto the piezoelectric transducer 50 can be optionally adjusted. Consequently, the resonance frequency of the piezoelectric transducer 50 can be varied according to the voltage applied by the direct-current power source 61. By controlling the direct-current voltage, the alternating current of the drive power source 63 is put in phase with the mechanical resonance frequency of the piezoelectric transducer 50, and ultrasonic vibration is efficiently raised.
However, the related-art piezoelectric transducer requires a fastening mechanism such as a bolt and nut combination. The structure of the piezoelectric transducer is thus limited. At the same time, the direct-current voltage should be applied to the piezoelectric element so as to control the resonance frequency of the piezoelectric transducer. The piezoelectric transducer must be mechanically highly precise to obtain a desired resonance frequency from its configuration. In the related art, the resonance frequency of the piezoelectric transducer cannot be adjusted freely in a wide range.
A known vibration control unit absorbs mechanical vibration by using a piece of rubber. Another known vibration control unit analyzes the phase, the amplitude and other conditions of the mechanical vibration, applies vibration of a phase opposite to that of the mechanical vibration to the mechanical component, and thus controls the mechanical vibration.
However, these related-art vibration control units fail to control a certain band of mechanical vibration occurring with the mechanical component.
For example, the former related-art vibration control unit using the piece of rubber can control the vibration in a wide frequency band. However, the vibration control unit cannot control the vibration of a specified small range of frequencies.
The related-art vibration control unit applying vibration from the outside requires a complicated structure so that the vibration control unit applies the vibration having a phase opposite to that of the mechanical vibration synchronously with the mechanical vibration. The vibration control unit requires an actuator for applying vibration from the outside, a sensor for detecting vibration, and a control circuit for executing high speed calculation. The vibration control unit thus requires an intricate, sophisticated, and large-sized structure, thereby occupying large space. However, the place where the vibration control unit is used is limited.
Conventionally, to obtain a desired frequency distribution, amplitude and other desired conditions of the mechanical vibration transmitted to a vibrating member, the configuration, material and other structural factors of the vibrating member are altered, and the vibrational characteristic of the vibrating member is adjusted.
However, in the above related art, the configuration, material and other structural factors need to be determined by means of calculation or actual measurement. It is thus difficult to adjust the frequency distribution and the amplitude of the mechanical vibration to a desired conditions.
The mechanical vibration transmitted to the vibrating member determines the structure of the vibrating member. When the vibrating member originally functions as a transmission, a support for other members, or the like, the design of the vibrating member is restricted within narrow limits.
At the same time, when the frequency distribution, the amplitude and other conditions of the mechanical vibration transmitted to the vibrating member vary, the vibrating member should be designed so that the vibrating member can correspond to variances of the vibration conditions. Such designing of the vibrating member is difficult. The vibrating member which can follow widely varied vibration conditions cannot be designed.